The present invention relates to the development of reconfigurable artificial magnetic conductor (RAMC) surfaces for low profile antennas. This device operates as a high-impedance surface over a tunable frequency range, and is electrically thin relative to the frequency of interest, λ.
A high impedance surface is a lossless, reactive surface, realized as a printed circuit board, whose equivalent surface impedance is an open circuit, which inhibits the flow of equivalent tangential electric surface currents, thereby approximating a zero tangential magnetic field. A high-impedance surface is important because it offers a boundary condition which permits wire antennas (electric currents) to be well matched and to radiate efficiently when the wires are placed in very close proximity to this surface (<λ/100 away). The opposite is true if the same wire antenna is placed very close to a metal or perfect electric conductor (PEC) surface. It will not radiate efficiently. The radiation pattern from the antenna on a high-impedance surface is confined to the upper half space above the high impedance surface. The performance is unaffected even if the high-impedance surface is placed on top of another metal surface. The promise of an electrically-thin, efficient antenna is very appealing for countless wireless device and skin-embedded antenna applications.
One embodiment of a thin, high-impedance surface 100 is shown in FIG. 1. It is a printed circuit structure forming an electrically thin, planar, periodic structure, having vertical and horizontal conductors, which can be fabricated using low cost printed circuit technologies. The high-impedance surface or artificial magnetic conductor (AMC) 100 includes a lower permittivity spacer layer 104 and a capacitive frequency selective surface (FSS) 102 formed on a metal backplane 106. Metal vias 108 extend through the spacer layer 104, and connect the metal backplane to the metal patches 110 of the FSS layer. The thickness of the high impedance surface 100 is much less than λ/4 at resonance, and typically on the order of λ/50, as is indicated in FIG. 1.
The FSS 102 of the prior art high impedance surface 100 is a periodic array of metal patches 110 which are edge coupled to form an effective sheet capacitance. This is referred to as a capacitive frequency selective surface (FSS). Each metal patch 110 defines a unit cell which extends through the thickness of the high impedance surface 100. Each patch 110 is connected to the metal backplane 106, which forms a ground plane, by means of a metal via 108, which can be plated through holes. The spacer layer 104 through which the vias 108 pass is a relatively low permittivity dielectric typical of many printed circuit board substrates. The spacer layer 104 is the region occupied by the vias 108 and the low permittivity dielectric. The spacer layer is typically 10 to 100 times thicker than the FSS layer 102. Also, the dimensions of a unit cell in the prior art high-impedance surface are much smaller than λ at the fundamental resonance. The period is typically between λ/40 and λ/12.
Another embodiment of a thin, high-impedance surface is disclosed in U.S. patent application Ser. No. 09/678,128, entitled “Multi-Resonant, High-Impedance Electromagnetic Surfaces,” filed on Oct. 4, 2000, commonly assigned with the present application and incorporated herein by reference. In that embodiment, an artificial magnetic conductor is resonant at multiple resonance frequencies. That embodiment has properties of an artificial magnetic conductor over a limited frequency band or bands, whereby, near its resonant frequency, the reflection amplitude is near unity and the reflection phase at the surface lies between +/−90 degrees. At the resonant frequency of the AMC, the reflection phase is exactly zero degrees. That embodiment also offers suppression of transverse electric (TE) and transverse magnetic (TM) mode surface waves over a band of frequencies near where it operates as a high impedance surface.
Another implementation of a high-impedance surface, or an artificial magnetic conductor (AMC), which has nearly an octave of +/−90° reflection phase, was developed under DARPA Contract Number F19628-99-C-0080. The size of this exemplary AMC is 10 in. by 16 in by 1.26 in thick (25.4 cm×40.64 cm×3.20 cm). The weight of the AMC is 3 lbs., 2 oz. The 1.20 inch (3.05 cm) thick, low permittivity spacer layer is realized using foam. The FSS has a period of 298 mils (0.757 cm), and a sheet capacitance of 0.53 pF/sq. The FSS substrate had a thickness of 0.060 inches, and was made using Rogers R04003 material. The FSS was fabricated using two layers of metallization, where the overlapping patches were essentially square in shape.
The measured reflection coefficient phase of this broadband AMC, referenced to the top surface of the structure is shown in FIG. 2 as a function of frequency. A ±90° phase bandwidth of 900 MHz to 1550 MHz is observed. Three curves are traced on the graph, each representing a different density of vias within the spacer layer. For curve AMC1-2, one out of every two possible vias is installed, and only the upper patches are connected to the vias. For curve AMC1-4, one out of every four vias is installed. In this case, only half of the upper patches are connected to vias, and the patches connected form a checkerboard pattern. For curve AMC1-18, one out of every 18 vias is installed. In this third case, only one in every 9 of the upper patches has an associated via. As expected from the effective media model described in application Ser. No. 09/678,128, the density of vias does not have a strong effect on the reflection coefficient phase.
Transmission test set-ups are used to experimentally verify the existence of a surface wave bandgap for this broadband AMC. In each case, the transmission response (S21) is measured between two Vivaldi-notch radiators that are mounted so as to excite the dominant electric field polarization for transverse electric (TE) and transverse magnetic (TM) modes on the AMC surface. For the TE set-up, the antennas are oriented horizontally. For the TM set-up, the antennas are oriented vertically. Absorber is placed around the surface-under-test to minimize the space wave coupling between the antennas. This optimal configuration—defined empirically as “that which gives the smoothest, least-noisy response and cleanest surface wave cutoff”—is obtained by trial and error. The optimal configuration is obtained by varying the location of the antennas, the placement of the absorber, the height of absorber above the surface-under-test, the thickness of absorber, and by placing a conducting foil “wall” between layers of absorber to mitigate free space coupling between test antennas. The measured S21 for both configurations is shown in FIG. 3. As can be seen, a sharp TM mode cutoff occurs near 950 MHz, and a gradual TE mode onset occurs near 1550 MHz. The difference between these two cutoff frequencies is referred to as a surface wave bandgap. This measured bandgap is correlated closely to the +/−90-degree reflection phase bandwidth of the AMC illustrated in FIG. 2.
The resonant frequency of the prior art AMC, shown in FIG. 1, is given by Sievenpiper et. al. (IEEE Trans. Microwave Theory and Techniques, Vol. 47, No. 11, Nov. 1999, pp. 2059-2074), (also see “High Impedance Electromagnetic Surfaces,” dissertation of Daniel F. Sievenpiper, University of California at Los Angeles, 1999) as f0=1/(2π√{square root over (LC)}) where C is the equivalent sheet capacitance of the FSS layer in Farads per square, and L=μ0h is the permeance of the spacer layer, with h denoting the height or thickness of this layer.
In most wireless communications applications, it is desirable to make the antenna ground plane as small and light weight as possible so that it may be readily integrated into physically small, light weight platforms such as radiotelephones, personal digital assistants and other mobile or portable wireless devices. The relationship between the instantaneous bandwidth, BW, of an AMC with a non-magnetic spacer layer and its thickness is given by       BW          f      0        =      2    ⁢    π    ⁢                   ⁢          h              λ        0            where λ0 is the free space wavelength at resonance where a zero degree reflection phase is observed. Thus, to support a wide instantaneous bandwidth, the AMC thickness must be relatively large. For example, to accommodate an octave frequency range (BW/f0=0.667), the AMC thickness must be at least 0.106 λ0, corresponding to a physical thickness of 1.4 inches at a center frequency of 900 MHz. This thickness is too large for many practical applications.
Accordingly, there is a need for an AMC which allows for a larger reflection phase bandwidth for a given AMC thickness. The approach taught in accordance with embodiments of the present invention is to permit the limited reflection phase bandwidth to be electronically reconfigurable.
One popular type of broadband antenna is a cavity backed spiral. This is a slot or wire planar spiral antenna installed over a metal cavity. If the cavity is filled with absorber, then the antenna “sees” only free space above it and the antenna's impedance bandwidth can extend over multiple octaves. The down side is that the antenna's efficiency has an upper bound of only 50% since power radiated into the absorber is wasted as heat. Alternatively, the cavity may be filled with a low loss dielectric material such that the electrical depth of the cavity is one-quarter of a wavelength at the center frequency. Foam or honeycomb are common dielectrics for this purpose, but this forces the antenna to be too thick and heavy for many low frequency applications. Dielectric loading of the cavity will decrease the thickness in proportion to the square root of the dielectric constant, but this forces surface waves or longitudinal section electric (LSE) and longitudinal section magnetic (LSM) modes to be excited in the cavity which create undesired parasitic resonances. Thus, there is a need is create a thin, lightweight substrate, which will not support surface waves, to permit the realization of a shallow cavity, broadband antenna.